Embodiments described herein relate to the field of geological mapping.
Active source electromagnetic (EM) surveying such as time domain electromagnetic (TDEM) surveying is a rapidly developing area of geophysical surveying. It encompasses ground based and airborne applications. TDEM geological mapping involves measuring the magnetic response of the earth to a primary magnetic field transmitted by the survey system. The relation between the transmitted primary field and the response is used to calculate the electrical resistivity structure of the earth, from which geological information is inferred.
Examples of TDEM surveying systems and methods are shown in U.S. Pat. No. 7,157,914 and WIPO patent applications PCT/CA2009/001197, PCT/CA2009/000217 and PCT/CA2010/001863.
Natural source electromagnetic surveying such as AFMAG surveying is another developing area of geophysical surveying, and is also used in ground and airborne applications. It involves measuring the magnetic response of the earth to naturally occurring electromagnetic fields. The transfer functions between various components of the magnetic field, some of which may be measured with a moving receiver and others at a fixed location, are used to calculate the electrical resistivity structure of the earth, from which geological information is inferred.
Examples of airborne AFMAG surveying systems and methods are shown in U.S. Pat. No. 6,876,202, U.S. patent application Ser. No. 12/910,386 published as US2011/0115489 A1, and U.S. patent application Ser. No. 12/645,915 published as US2010/0244843 A1.
In a practical EM survey system, the secondary response of the earth is sensed by one or more receivers (which may be mutually orthogonal) and is recorded by a data acquisition system. In an active source system, the effect of the primary magnetic field on the receiver may be reduced by the use of a bucking coil to allow for more accurate recording of the secondary field. Especially when a bucking coil is present, the transmitted primary magnetic field may also be sensed and recorded by a current sensor.
In the case of an active source system, the recorded transmitter current waveform and the secondary magnetic field waveforms are affected by the transmitted primary magnetic waveform, the bucking coil waveform, and the transfer functions of the receivers and recording system. While these effects tend to be small at long delay times (low frequencies) they become more important at short delay times (high frequencies). In the case of natural source systems, the amplitude and phase of the signals are affected by the receivers and recording system.
In order to properly interpret the recorded waveforms, it is desirable to reduce or remove the effects of the transmitted primary magnetic waveform, the receivers, and the recording system. This requires that the impulse response (or transfer function) of the sensors and recording system be precisely known. Since the impulse responses and transfer functions vary (for example, due to temperature changes) it is desirable to measure them at appropriate intervals, in field conditions.
The receivers in ground based natural source EM systems are typically ferromagnetic core magnetic induction sensors with a solenoid shape. Such receivers can conveniently be provided with a calibration winding in the form of a solenoid which encloses the main winding, extends slightly beyond the ends of the core, and is comparable in size to the sensor under calibration. This takes advantage of the fact that the magnetic field inside a solenoid winding can be accurately calculated knowing only the pitch of the winding and the current flowing in it.
The receivers in TDEM systems and airborne natural source EM are typically air coil magnetic induction sensors in the form of loops having multiple turns of wire. Such loops may be on the order of 1 m to 8 m in diameter, for example. Large air coil receivers have advantages including cleaner time domain response and a more convenient shape for airborne towing. In a field survey environment, it is difficult to provide a magnetic field of precisely known waveform and amplitude over a volume large enough to contain such a receiver.